"Comments may only be edited for 5 minutes."

Question

Why?

Is going back and later editing comments later to say something else a problem? If I make a spelling mistake and notice it, I can delete the comment and rewrite, but why not just leave it editable?

Answer 1

The five-minute window is a compromise.

1. Comments shouldn't be editable because StackExchange doesn't want comments to be "first-class citizens." Perpetual editing would highly encourage viewable histories, to "keep you honest." Which would in turn encourage treating comments less and less like ephemera.

2. Comments should be editable because it sucks when you press "post comment" by accident, or when you post a comment and then immediately notice a typo while you're rereading it.

In the designers' view, (1) is a much bigger deal than (2), so the compromise is a five-minute window to allow you to clean up small problems easier (without having to copy/paste repost).

Answer 2

When you have a question about core site features, it's worth checking on Meta Stack Exchange to see if there's an answer available; there is in this case.

Is going back and later editing comments later to say something else a problem?

Yes. Consider this comment exchange:

Barbarians are great! They can pounce which is totally awesome. — Jonathan Hobbs

I totally agree with @JonathanHobbs. — Jack Lesnie

Then I edit my comment the next day.

Barbarians are the stupidest class ever and pounce is stupid and whoever wrote them is stupid. — Jonathan Hobbs

I totally agree with @JonathanHobbs. — Jack Lesnie

Comments don't have a revision history, so you can't tell what I wrote beforehand. Originally, comments couldn't even be edited at all — the 5 minute edit window was added so you could fix typos, and 5 minutes is long enough to do that. It's also short enough others usually won't be replying to your comment inside of the edit window.

If you have a couple of spelling mistakes in your comment a while down the track, people can probably still get what you mean. If it's sufficiently embarrassing or a significant miscommunication, at least that's a good excuse for clean-up of the comment chain!

Answer 3

The reason, I suspect, is because it's entirely possible to say something you expect will gather votes of 'useful', and then edit the comment to say something else you want inserted into the answer. The votes will stay, and the now wrongly upvoted comment's content might be edited into the answer. (Of course, this requires un-savvy viewers.)